Fix Docker registry not working with Windows layers

Docker v2.7.0 changed the behavior from v2.6.2 in that it enabled the validations to check manifest URLs by default, which caused the registry to set the allowed URL regexp to nothing (^$) instead of nil. This caused the container registry to reject any foreign layers that contained URLs. We fix this by disabling the validations with the newly-introduced disabled flag.

See https://github.com/docker/distribution/issues/2795 for more details.

Closes #4013 (closed)